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Monday, 15 December 2014

Variation in clutch size in relation to nest size in birds. Moller, Ecology and Evolution: December 2014

Variation in clutch size in relation to nest size in birds.
Anders P. Møller et al

Ecology and Evolution 2014; 4(18): p3583/95
doi: 10.1002/ece3.1189

Laboratoire Ecologie, Systematique et Evolution, UMR 8079 CNRS-Universite Paris-Sud XI-AgroParisTech, Batiment 362 Universite Paris-Sud XI, Orsay Cedex F-91405, France E-mail:


There are six main, non-mutually exclusive, reasons why large nests may be advantageous. 

First, the maintenance of a specific nest temperature and humidity. Optimal nest size may provide a microenvironment that is not too cooled or too over-heated for growth, and development of nest temperature regulation.
Second, specific types of nest material may provide protection of eggs and nestlings against bacteria and parasites.
Third, an optimal nest size will prevent excessive fouling of nests and the associated fitness costs of nestling death by allowing parents to keep the nest clean.
Fourth, sexual selection: nest size can almost double when both male and female build nests as compared to nests built by females alone, because both parents signal their phenotypic quality via their investment in nest building.
Fifth, structural support may prevent eggs and nestlings from being lost.
Sixth, large nests may reduce crowding of the offspring and prevent them from falling out of the nest.

Nest sizes are restricted by five main selective forces. 

First, the risk of nest predation and parasitism selects for smaller nest sizes by disproportionately affecting larger nests.
Second, building larger nests means more time spent collecting nesting materials, which are often found on the ground, increasing the risk of predation on nest-building adults.
Third, large and consequently warm and humid nests do not only benefit offspring, but also pathogens and parasites that inhabit nests, and bacteria multiply at higher rates in warm and humid nest environments compared to ambient temperatures, with potential costs to their avian inhabitants.
Fourth, nest size may be limited by individual quality, and the construction of larger nests when males participate in nest building may have a signalling function, reflecting the working ability of males for finding specific or scarce nest materials.
Fifth, nest size may increase the risk of brood parasitism, or decrease the probability of rejection of host offspring by brood parasites.

Study Objectives
There are a few factors that determine nest size including laying date and female nest-site selection however the most significant determinant is clutch size. Altering the nest size during egg-laying has shown a direct correlation with subsequent clutch size therefore the objective of this study was to investigate clutch size variation in response to nest size in both hole and open nesting birds.
First objective: "The effect of nest box size on clutch size" Using data from Great Tit studies where differently sized nest boxes were available, and when the nest box size was altered after laying had started.
Second objective: "The ratio of clutch size to nest size in hole nesters." Using nest boxes of various sizes within a population.
Third objective: "The relationship between clutch size and nest size in secondary hole-nesting species" Using four species with access to both single or multiple nest box sizes to find a predicted homogeneous relationship linking clutch size to cavity size.
Fourth objective: "Inter-species response to cavity size." Populations in different environments may show variation in their clutch size response to cavity size.
Fifth objective: "Latitude and longitude effect on clutch size and nest size." Climatic conditions may underly the variation in clutch size across studies.

Broadening the analysis to a larger number of species, including open nesting species as these are less constrained by nest cavity size, and correcting for geographic and habitat patterns, there should be enough evidence available to observe a correlation between clutch size and nest cavity size. 

Materials and Methods
Data sets 
Löhrl’s (1973, 1980) data on Great Tits (choice of nest box and experimental change of box size during laying). (Löhrl, H. 1973. Einfluß der Brutraumfläche auf die Gelegegröße der Kohlmeise (Parus major). J. Ornithol. 114:339–347. Löhrl, H. 1980. Weitere Versuche zur Frage “Brutraum and Gelegegröße” bie der Kohlmeise (Parus major). J. Ornithol. 121:403–405.) 

Database on clutch size and nest box size in four secondary hole-nesting birds. 155 study populations of Great Tits for 1479 effective study years, 121 of Blue Tits for 1122 esy, 24 of Pied Flycatchers for 288 esy, and 65 of Collared Flycatchers for 592 esy; in total 79,610 clutches. (Møller, A. P., F. Adriaensen, A. Artemyev, J. Banbura, E. Barba, C. Biard, et al. 2014. Clutch size in European secondary hole-nesting passerines in relation to nest-box floor area, habitat, geographic location and study year. Methods Ecol. Evol. 5:353–362.) 

Database on the relationship between clutch size and nest size in 21 species. Nest size in hole-nesters was defined as the two-dimensional area of the nest box, or nest hole in case the study was based on natural cavities; in open nesting species the horizontal maximum area of the nest included nest cup and rim. For Great Tits studied with increasing available nest box sizes the mean nest base area weighted by sample size across all 3447 samples was 118 cm2, range 36–400 cm2, SD = 123 cm2.
Environmental cofactors measured included; latitude (°N) and longitude (°E), main habitat type (deciduous, coniferous, evergreen, or mixed), urbanisation (urbanised, or natural/ semi-natural habitat), altitude at the centre of a study plot, mean study year, nest floor surface as the internal nest base area, and the material used to construct nest boxes (wood or concrete), minimum and maximum size of boxes. 

Statistical analyses were performed to examine the relationship between clutch size and nest size within and among species of birds by analysing an exhaustive compilation of data on both hole and open nesting species and a wide range of cofactors. 

Clutch size and nest size in Löhrl’s studies of Great Tits: An increase by one standard deviation in nest base area (through choice of either 64 or 314 cm2) was associated with an average increase in clutch size of between 1.08 - 3.25 eggs.
Clutch size and nest size across four species of birds (Great Tit, Blue Tit, Pied Flycatcher, Collared Flycatcher): The relationship between clutch size and base area in populations with one or more box sizes for the four species was statistically significant for all species combined. It was also statistically significantly positive for Great Tit separately, but not for the other three species.
The interaction between species and nest floor area was highly significant (P < 0.0001), implying that clutch size was related to nest floor area in a species-specific manner. Testing for a species-specific relationship between clutch size and nest base area based on study sites where more than one box size was used and more than one nest size was recorded (Fig. 1) showed a significant difference in means and variance (Fig. 2). Adding longitude or latitude into the statistics indicated that these had not biased the data.

From these data most populations and species of both hole and open nesters showed increased clutch size with increased nest size, with variation within and among species. Competition for nesting sites is increasing in this age of managed forests and loss of suitable wild habitats and will therefore have the effect of reducing numbers of fledged birds, a possible contributor to the recent decline in bird numbers. Conservation organisations that seek to promote the use of bird boxes should investigate whether an increase in the size of box would be beneficial in the long term.

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